1. Field of the Invention
The present invention relates to a resin-coated carrier for an electrophotographic developer used for a two-component electrophotographic developer for use in copying machines, printers and the like, and an electrophotographic developer using the carrier.
2. Description of the Related Art
In order to improve the durability of carriers for two-component developers, there have been reported resin-coated carriers coated with various types of resins as a toner spent-preventive measure.
However, by coating a carrier with a resin, the resistance of the carrier increases, and there arise problems of causing a decrease in image density and a decrease in image quality such as an edge phenomenon. The resistance value of a carrier must be optimized according to machine systems and development conditions. As the carrier resistance adjustment method, addition of a conductive substance (conductive agent) to a coating resin layer has been reported a lot. As usual conductive substances, various types of carbon black are broadly known because they are inexpensive and allow easy adjustment of the resistance.
Although for color toners, particularly light-colored toners (yellow toners and the like), a carrier whose resistance is adjusted by addition of carbon black to a coating resin has no problem with the image density and the edge phenomenon, but there arises a problem that the carbon black added to the coating resin is mingled in the toner and makes the color turbid, thereby decreasing the image quality. Also in regard to the environmental dependency, carbon black itself has so low resistance that a carrier using the carbon black largely depends on resistance, thereby causing charge leakage particularly at a high temperature and a high humidity; thus, when a power source of a machine is turned on, scumming is liable to occur due to intense discharge and charge rise is poor, thereby resulting in that a clear image cannot be obtained. Thus, the carrier using the carbon black has a problem with the environmental dependency.
Conversely, if the amount of carbon black added is reduced in order to improve the color smear and the environmental dependency, a decrease in the image density and a decrease in the image quality such as an edge phenomenon are caused.
In order to simultaneously meet measures to a decrease in the image density, a decrease in the image quality such as an edge phenomenon, the color smear and the environmental dependency as described above, the following proposals have been made. That is, in Japanese Patent Laid-Open No. 2004-45773, a carrier for an electrophotographic developer is proposed in which the particle surface of a magnetic carrier core material is provided with a coating resin layer containing fullerene and/or a carbon nanotube.
Japanese Patent Laid-Open No. 2009-210795 proposes a carrier for electrophotography having at least a core material particle and a coating layer which coats the core material particle, in which carrier for electrophotography the coating resin layer contains at least a binder resin, a solid particle and a carbon nanotube; and if the particle diameter of the solid particle is taken as D (μm) and the average thickness of the resin portion in the coating layer is taken as h (μm), 1<(D/h)<10 is satisfied.
However, it is the present situation that in the above-mentioned proposals, a coating layer is shaved due to long-time endurance printing and color smear thereby occurs, and thus the above-mentioned proposals do not provide any drastic measures.
In recent years, studies in high-charge amount areas have been made in order to make a toner particle diameter small along with the requirement of high image quality. Also along with image quality enhancement and life elongation of carrier core materials, the carrier core materials have moved from high-magnetic force core materials such as iron powder to low-magnetic force core materials such as ferrites, thereby causing resistance as core materials to increase; as a result, the resistance of developers is too high by conventional methods, which causes a decrease in the image density and the occurrence of the edge phenomenon, thus causing a problem that a desired image quality and life cannot be obtained. If the amount of a conductive substance added is increased to adjust the resistance, the strength of coating resins decreases, thereby resulting in further shortening the life.